Ever since the debate about same sex marriage was launched last year I've been wondering what all the fuss is about.

Obviously, opening what has previously been an exclusively heterosexual legal status to couples of the same gender was going to cause some controversy. There was opposition from traditionalists to the creation of civil partnerships in 2004, though you wouldn't know it to listen to most of them today. Allowing gay couples to adopt children was once controversial. So was equalising the age of consent at 16. So was reducing the age of consent for same sex partners from 21, which happened as rececently as 1994. These measures always attract opposition, often from the churches. 

But the opposition to same sex marriage has been of a different order. Some of it has been quite bizarre.

600,000 people have signed an online petition opposing any "redefinition" of marriage to include gay couples. MPs' postbags have bulged, swelled by a write-in campaign preached from the pulpits of many churches. Senior Tory figures have warned of mass defections of party members. Religious leaders have spoken in apocalyptic terms of what will befall society should the proposed change go ahead. Scotland's Cardinal Keith O'Brien suggested that the move was akin to legalising slavery. One openly gay Conservative, Conor Burns, has spoken of his shock at the tone of some of the correspondence [2]: "I don’t know what kind of God some of those people who have contacted me from religious groups believe in," he said, "but he’s certainly not compassionate or loving."

Around 50 MPs voted against civil partnerships in 2004. It was reported at the weekend that up to 180 Conservative MPs may rebel (on a free vote) in today's debate. We shall see. But at a time when poverty is rising, the economy - to put it politely - becalmed and the NHS, the education system and the police in organisational chaos, you have to wonder precisely why for so many people same sex marriage has become such a big deal.

It's worth remembering that for all the heated debate the proposed change is almost entirely cosmetic. Civil partnership already gives a couple all the legal rights of marriage, as opponents of the Same Sex Couples Bill never tire of pointing out. All they lack is the ability to say that they are legally married. How can a single word be so important?

The nearest thing to a coherent argument against same sex marriage goes something like this. The majority of human cultures have always seen in the union of a man and a woman the basic building-block of society. Marriage isn't merely an arrangement between the couple, but anchors the wider social realm - and, like it or not, homo sapiens comes in two distinct sexes (with some fuzziness around the edges, perhaps, but let's not complicate things too much). Above all, marriage is about the family, about the raising of the next generation. Therefore it is rightly regarded as in some sense sacred. Its heterosexual nature is part of its intrinsic nature: some go on to argue that "same sex marriage" is an oxymoron, or at best a legal fiction.

But if marriage is mainly about children, what about marriages which are infertile, or where the couple is past childbearing age, or where one of the partners is terminally ill or incapacitated? In none of these cases do proponents of traditional marriage object. These are said to be exceptions to the general rule. What they fail to explain is why a marriage where the partners are of the same sex might not equally be considered an exceptional case. At present, more marriages take place between heterosexual couples who intend not to have children than are ever likely to take place between gay couples.

If campaigners for "traditional marriage" care so much about formalised, monogamous heterosexual relationships, should they not be concentrating their efforts on preventing marriage breakdown, perhaps by making divorce more difficult, than on frustrating the desire of a relatively small number of same-sex couples to tie the knot? To be fair, some do campaign on more general issues around the state of marriage - but rarely, it seems to me, with anything like the intensity they bring to bear on their obsession with same-sex unions. 

I've come to the reluctant conclusion that same sex marriage inspires such passionate opposition mainly because it represents the last symbolic move towards full equality of esteem for gay couples. Legal equality they already have, or close enough: civil partnerships represent the substance of marriage without the name. But the name matters, because without it those who have never quite been reconciled to gay rights can convince themselves that heterosexual marriage is still a qualitatively different from and superior to its homosexual equivalent. While the law reflects this distinction, all is not lost: opposite sex marriage is still special, therefore heterosexuality is still special, still normative rather than merely the majority preference. 

Civil partnerships, however unwelcome they seemed at the time (and the Catholic Church still opposes them) are quite useful in this regard. On the one hand, they can be held up as evidence that same sex marriage is unnecessary, a symbolic distraction, because gay couples now have legal rights. (And also as evidence that the speaker is not homophobic, because "I support civil partnerships".) But on the other, the distinction between the two forms of public union reinforces, perhaps even creates, the idea that heterosexual and gay relationships are different types of thing. 

It would be untrue to say that support for civil partnership is the last refuge of homophobia in modern Britain; gay people still face much more serious forms of hatred and discrimination. It's a sign of progress that the section of religious and political opinion that 45 years ago fought the legalisation of gay sex contents itself today with arguing about the meaning of a word. But the vehemence of the opponents of equal marriage, out of all proportion to any effect that the proposed change could possibly have, suggests a darker motivation than they admit or even realise.